1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a facsimile, and a copier.
2. Description of the Related Art
It is important for image forming apparatuses to make it possible to stably provide desired image density. For this reason, many image forming apparatuses form a density detection pattern made of a non-transferred toner not to be transferred onto a sheet, on an image carrier. The image forming apparatuses detect the image density of the density detection pattern with an optical detecting unit. Based on the detection result, the image forming apparatuses adjust image forming conditions (specifically, toner density, LD power, a charging bias, and a developing bias, for example), thereby performing density adjustment control for adjusting the amount of adhering toner to a target amount.
An image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2001-305823 forms a density detection pattern in an inter-sheet area on an intermediate transfer belt serving as an image carrier. The inter-sheet area corresponds to a space between a trailing end of a preceding sheet and a leading end of a following sheet in continuous image formation. The image forming apparatus then detects the density detection pattern to perform density adjustment control. The image forming apparatus feeds a sheet into a secondary transfer area in which a secondary transfer member and the intermediate transfer belt face each other with the secondary transfer member brought into contact with the intermediate transfer belt to transfer an image formed on the intermediate transfer belt onto the sheet.
To prevent the sheet from being stained with a toner forming the density detection pattern, the density detection pattern formed in the inter-sheet area on the intermediate transfer belt is formed so as not to come into contact with the sheet in the secondary transfer area. Thus, no sheet is present in the secondary transfer area while the density detection pattern is passing through the secondary transfer area. As a result, the density detection pattern comes into contact with the secondary transfer member facing the intermediate transfer belt in the secondary transfer area. This causes the surface of the secondary transfer member to be stained with the toner forming the density detection pattern. If no countermeasure is taken, the toner adhering to the surface of the secondary transfer member adheres to the back of a sheet subsequently fed into the secondary transfer area, thereby causing a stain on the back of the sheet.
An image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2001-312154 is provided with a cleaning device that cleans the surface of a secondary transfer roller serving as a secondary transfer member. The cleaning device includes a brush member or a blade member provided to be in contact with the surface of the secondary transfer roller. The image forming apparatus scrapes off and removes a toner adhering to the surface of the secondary transfer roller with the brush member or the blade member, thereby preventing a stain on the back of the sheet.
The density adjustment control may be performed at a timing after the power is turned ON or a timing after a print job is finished and with a regular interval. To stabilize the image density in continuous printing, the density adjustment control may be performed during a continuous print job (refer to Japanese Patent Application Laid-open No. 2001-312154). To perform the density adjustment control during a continuous print job, a density detection pattern is formed in an inter-sheet area on an intermediate transfer belt corresponding to a space between a trailing end of a preceding sheet and a leading end of a following sheet in the continuous printing.
In an image forming apparatus using an intermediate transfer method using an intermediate transfer belt, in order to improve the sheet adaptability and the accuracy of the image position, a density detection pattern formed on a photosensitive element is primarily transferred onto the intermediate transfer belt and then the density detection pattern on the intermediate transfer belt is detected to perform the density adjustment control.
The image forming apparatus using the intermediate transfer method feeds a sheet into a secondary transfer area between a secondary transfer roller serving as a secondary transfer member and the intermediate transfer belt to secondarily transfer an image formed on the intermediate transfer belt onto the sheet. To prevent the sheet on which the image is to be formed from being stained with a toner forming the density detection pattern, the density detection pattern formed in an inter-sheet area on the intermediate transfer belt is formed so as not to come into contact with the sheet in the secondary transfer area. Thus, no sheet is present in the secondary transfer area while the density detection pattern is passing through the secondary transfer area. As a result, the density detection pattern comes into contact with the secondary transfer roller facing the intermediate transfer belt in the secondary transfer area. This causes the secondary transfer roller to be stained with the toner forming the density detection pattern. If no countermeasure is taken, the toner adhering to the secondary transfer roller adheres to the back of a sheet subsequently fed into the secondary transfer area, thereby causing a stain on the back of the sheet.
To improve the performance for separating the sheet that passes through the secondary transfer area from the secondary transfer roller, a roller member with a relatively small diameter may be used as the secondary transfer roller. It is difficult, however, to clean the surface of such a thin roller member with the brush member or the blade member. Because a large amount of toner of the density detection pattern adheres to the surface of the secondary transfer roller, the cleaning device may possibly fail to sufficiently remove the toner adhering to the surface of the secondary transfer roller. As a result, the stain on the back of the sheet may possibly be caused.
An image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2007-286176 separates a secondary transfer roller from an intermediate transfer belt while a density detection pattern formed in an inter-sheet area on the intermediate transfer belt is passing through a secondary transfer area. This causes the secondary transfer roller to be separated from the intermediate transfer belt in the inter-sheet area in which no sheet enters the secondary transfer area. As a result, the density detection pattern formed in the inter-sheet area on the intermediate transfer belt is prevented from adhering to the surface of the secondary transfer roller. This makes it possible to prevent a stain on the back of the sheet caused by the density detection pattern formed in the inter-sheet area on the intermediate transfer belt.
The stain on the back of the sheet caused by adhesion of the toner to the surface of the secondary transfer roller is not necessarily attributed to the density detection pattern formed in the inter-sheet area on the intermediate transfer belt. In recent years, there has been developed an image forming apparatus that performs what is called borderless printing for forming an image to an end of a sheet such that there is no margin at the edges of the entire perimeter of the sheet or any of the edges of the sheet (refer to Japanese Patent Application Laid-open No. 2004-45457). To form an image to an end of a sheet in borderless printing, a toner image simply needs to be formed on an intermediate transfer belt so as to be aligned with the position of the end of the sheet fed into a secondary transfer area. However, the formation of the toner image is difficult to carry out because not a little deviation occurs in the conveyance position of the sheet entering the secondary transfer area. Thus, the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2004-45457 forms an image on the intermediate transfer belt such that the image protrudes from the edge of the sheet.
Using a piece of thick paper as the sheet increases a shock of a collision of the sheet with the secondary transfer roller when the sheet is fed into the secondary transfer area in which the secondary transfer roller is brought into contact with the intermediate transfer belt compared with the case of using a piece of thin paper. The shock causes large vibration on the secondary transfer roller, and the vibration is transmitted to a photosensitive element in an image forming unit and an exposing device. As a result, a position at which the exposing device writes a latent image on the photosensitive element may possibly deviate from a normal position, resulting in deterioration in image quality.
The inventors of the present application conducted earnest research and found the following. Using a foam roller obtained by covering a cored bar with foam as the secondary transfer roller enables the foam to mitigate a shock of a collision of a sheet with the foam roller. This reduces vibration caused by the collision, thereby suppressing the deterioration in image quality.
However, a plurality of holes are formed on the surface of the foam roller, thereby making it difficult to physically remove a toner entering the holes with a brush or a blade. Thus, if the surface of the foam roller is stained with the toner forming the density detection pattern, the brush member or the blade member of the cleaning device fails to sufficiently remove the toner from the surface of the foam roller. As a result, the stain on the back of the sheet is caused.
While the explanation has been made of the stain on the back of the sheet made in the case where the density detection pattern is formed in the inter-sheet area on the intermediate transfer belt to perform density adjustment control, the non-transferred toner not to be transferred onto the sheet is not necessarily attributed to the density detection pattern. Examples of adhesion of the non-transferred toner to the inter-sheet area on the image carrier may include forcible ejection of a toner in a developing unit to the inter-sheet area on the image carrier to replace a deteriorated toner in the developing unit by a new toner. In this case as well, problems similar to those described above occur.
Forming an image so as to protrude from an edge of a sheet causes a toner protruding from the edge of the sheet to adhere to the intermediate transfer belt. The protruding toner adheres from the intermediate transfer belt to the surface of the secondary transfer roller when the image is transferred from the intermediate transfer belt onto the sheet in the secondary transfer area. As a result, the toner adhering to the secondary transfer roller adheres to the back of a sheet subsequently fed into the secondary transfer area. Thus, the protruding toner causes a stain on the back of the sheet.
The problems similar to those described above may possibly occur in an image forming apparatus using a direct transfer method that directly transfers an image from a latent image carrier, such as a photosensitive element, onto a sheet not via an intermediate transfer body.
In view of the above, there is a need to provide an image forming apparatus that can prevent a stain on the back of a sheet caused by a non-transferred toner caused to adhere to an inter-sheet area on an image carrier.